In IC technology there is a need to stack chips together to form multi-tiered (3-D) IC devices (also referred to as multi-layered IC devices or stacked IC devices). One challenge that arises when chips are stacked is that thermal conductivity is reduced. Thus, hot spots could exist with little ability to move the heat away from the heat source. Because of the reduced size of stacked ICs, (substrate thickness going from 100 microns to about 6-50 microns), the power density rises while lateral thermal conductivity is reduced.
One method for increasing lateral thermal conductivity is to increase the substrate thickness. This, in turn, negatively impacts the desired form factor of the stacked IC device and degrades performance.
An additional challenge exists when more than two tiers are stacked. In such situations, the stacked IC device may contain multiple layers of oxide, one between each pair of stacked tiers. Oxide, being a poor thermal conductor, adds to the heat dissipation problem.
There are several approaches for addressing the thermal conductivity issues. One approach positions a heat conducting layer between the tiers. Typically, heat conducting layers are metallic and thus could interfere with inter-layer electrical connections. Another approach uses Through Silicon Vias (TSVs) to move the heat from an inner tier to a surface tier of the stacked IC device and then remove the heat from the surface tier using traditional methods, such as positioning a high thermal conductivity material on the surface tier. Challenges arises with such a solution. For example, it is not always possible to position a TSV at the necessary location because of circuitry layout requirements in the various tiers.
Another approach is to circulate cooling material through the stacked IC device to cool the hot spots. A cooling circulation solution is costly to manufacture and, because of the moving liquid, requires a pumping mechanism and tight tolerances for liquid channels. Also, because of circuitry layout requirements, it may not be possible to “plumb” the device to channel the cooling material to the necessary location. The plumbing problem could be overcome, to some extent, by forcing cooling liquid through the substrate itself, but this method is not without a further set of problems and costs.